This invention relates to overbased calcium sulfonates, and more particularly to an improved process for continuously preparing overbased calcium sulfonates which are used as detergent and reserve alkalinity in lubricating oil additives.
Highly basic calcium sulfonates are commonly used in lubricating oils; the materials consist of a colloidal dispersion of amorphous calcium carbonate in oil. A calcium sulfonate surfactant acts to disperse and stabilize the calcium carbonate. When used as an automotive crankcase lubricant additive, these materials function to neutralize acids formed during normal engine operation. Thus, the solubilized base performs an essential function, neutralizing acidic deposit precursors and sulfur acids originating from both combustion by-products and lubricant oil oxidation.
Highly basic calcium sulfonate lubricant additives are prepared from oil-soluble sulfonic acids (M.W. ca 350-600) obtained from the sulfonation of either high boiling petroleum distillate fractions or synthetic alkylenzenes. In a hydrocarbon medium, alkaline earth sulfonates are known to form inverted micellar aggregates, and contain a polar core of solubilized inorganic base (calcium carbonate and calcium hydroxide) stabilized by an adsorbed layer of the calcium sulfonate surfactant. Through a process known as "overbasing", the amount of solubilized base may be increased to as high as 30 moles of base per mole of alkaline earth sulfonate.
Overbased calcium sulfonates are generally produced by carbonating a hydrocarbon solution of a "neutral" calcium sulfonate or sulfonic acid, a calcium base (usually calcium hydroxide and/or calcium oxide), and certain reaction promoters such as lower alcohols, especially methanol, and/or calcium chloride. During the reaction the calcium base reacts with gaseous carbon dioxide to form calcium carbonate. The calcium carbonate is incorporated into small inverted micellar structures during this process. These basic calcium sulfonate micelles effectively render the calcium base oil soluble, an essential step for this product to be useful in lubricating oil formulations. The final product must be filtered or centrifuged to remove unreacted solids, particularly since an excess of the calcium base is used in the production of highly basic sulfonates.
Economically, it is useful to obtain a product which is as highly basic as possible. However, as the basicity increases, product viscosity tends to increase and filtration rates decrease. Thus, an object of the present invention is to provide a continuous process to prepare overbased alkaline earth sulfonates (300-500 TBN range) having acceptable filterability and viscosity.